Influence of Patient Weight and Couch Rotation On the Accuracy of a Surface Tracking System for Stereotactic Radiosurgery

Purpose: To evaluate the influence of patient weight and couch angle on the treatment accuracy of brain SRS guided by the AlignRT system integrated with TrueBeam couch system.Method and Materials: A Winston Lutz phantom with a facemask on top was used to simulate a brain SRS treatment with surface monitoring. The phantom contained small fiducial markers that guided the initial setup using kV and CBCT imaging. A CT scan of the phantom was taken and used to form a plan that included 15 sets of different combinations of couch and gantry angles. Once the phantom was set up, a reference image for surface was acquired and motion monitoring was initiated. The gantry and couch were then moved to their respective positions and an EPID image was acquired. A motion report was generated on the AlignRT system. EPID image registration was performed using the OBI system with DRRimage and the shifts were recorded. The method was repeated for evaluation of patient weight with five different weights in the range of 20 and 120 kg. Results: Examination of the AlignRT data suggests that the accuracy of TrueBeam couch system is slightly compromised for the longitudinal direction (< 0.9 mm) after applying couch rotation during the tracking. The accuracy did not seem to be affected by varying patient weight if couch sagging is compensated for initial setup. All of the deviations of the AlignRT record showed that the accuracy was within 1 mm to the deviations from image registration of isocenter.Conclusions: Our data suggests that TrueBeam couch system maintains geometric accuracy of up to 120 kg of weight, at various gantry and couch angle positions, suggesting that the system is reliable for use with Brain SRS treatments. Addition of AlignRT for intrafraction monitoring helps to ensure that the treatment remains accurate.